Lava Slime

Lava Slimes are ordinary Underworld spawns, so you meet them the first time you dig that deep, before Hardmode and with no condition attached. What is different is their relationship with the terrain. Lava does not hurt them and they float on it the way a normal slime floats on water, except the surface flings them: instead of bobbing gently they get launched high into the air. They also jump about twice as high as the slimes on the surface and move faster than the family usually does, so they arrive from above and from further off than you expected. Like other slimes they ignore you completely if you have Royal Gel equipped.

In Expert and above every one of them leaves a small pool of lava exactly where it died, and that pool normally burns the coins it just dropped. Standing close as it dies is the fix, since an item already in the process of being collected cannot be destroyed. The volume varies from a splash to a full tile. About three times in four it is deep enough to scoop with a bucket, which is a convenient way to carry lava around without hauling it up from the floor of hell.

Those puddles are why a long farming session slowly ruins a solid walkway. Leave single tile gaps in the bridge so lava drains through instead of pooling on top, and cover those gaps with obsidian, stone or ash wood platforms, none of which melt. Skip that and you come back to a bridge burning in six places with your loot sitting underneath it.

A Guide Voodoo Doll lying on the floor can burn in a puddle one of these left behind, which summons the Wall of Flesh whether you were ready or not. It is rare, and it is reason enough to keep the doll in your inventory while you are down there. These also drop no gel at all, only coins, and the Remix and Zenith seeds flip that: no lava on death, gel instead.